Determinants of serum cotinine and hair cotinine as biomarkers of childhood secondhand smoke exposure

Abstract

Understanding the determinants of childhood secondhand smoke (SHS) exposure is important in measuring and preventing exposure to this widespread environmental contaminant. We evaluated the ability of a broad set of factors to explain variability in serum cotinine, reflecting recent exposure, and hair cotinine, reflecting longer-term exposure. We included repeated measures from 223 elementary-school-age asthmatic children residing with a smoker. We used a manual model-building approach and likelihood ratio tests to select a model predicting each biomarker, and also compared the predictive ability of determinants using Akaike Information Criteria. Potential determinants included a comprehensive parent questionnaire, household nicotine, home ventilation characteristics, exposure in vehicles and others' homes, child demographics, and family social class. Variables in each of these categories remained in the final model for both serum (R(2) of 0.61) and hair cotinine (R(2) of 0.45). A comprehensive set of factors was required to best predict cotinine. Studies should use biomarkers for the best quantitative assessment of SHS exposure. Hair cotinine may be a problematic measure because it was highly influenced by racial differences that were unexplained by SHS exposure. When biospecimen collection is not possible, a household nicotine measurement is warranted. If only questionnaires are available, multiple questions are required to best characterize exposure, such as number of cigarettes, hours spent in a room with concurrent smoking, maternal smoking, and approximate home size.

Figure 1

Predictive ability of increasingly complex models of determinants of serum cotinine and hair cotinine. R² values were estimated from ordinary least squares linear regression using data from the 12-month visit of the Cincinnati Asthma Prevention Study (2000–2003). Akaike Information Criterion values for models predicting serum cotinine (S) and hair cotinine (H) were estimated from linear mixed models using all available study data. Models included variables that met inclusion criteria as listed in Table 3, from the following additional groups: PQ, parent questionnaire, E, environmental measures, HV, housing and ventilation characteristics, NH, non-home places, Child, child characteristics, SES, family social class characteristics.